A solvothermal approach for the preparation of nanostructured carbide and boride ultra-high-temperature ceramics

Article abstract of DOI:10.1111/j.1551-2916.2010.04007.x

The use of a solvothermal process for the synthesis of tantalum carbide (TaC) and lanthanum hexaboride (LaB₆) powders in fused-quartz test tubes is reported in order to demonstrate the synthesis of these powders using thermal and chemical ignition techniques and to prove that the process is of a self-propagating high-temperature synthesis type, obviating the need for an autoclave. X-ray powder diffraction showed phase pure powders with crystallite sizes of 25 and 80 nm, while dynamic light scattering showed average particle sizes of 97 and 130 nm, for TaC and LaB₆, respectively. The data demonstrates that the powders have a very low level of agglomeration. Scanning electron microscopy shows that the TaC powders have a spherical morphology, while the LaB₆ powders have a mixture of cubic and spherical morphologies.

Full text of DOI:10.1111/j.1551-2916.2010.04007.x

J. Am. Ceram. Soc., 93 [10] 3035–3038 (2010)
DOI: 10.1111/j.1551-2916.2010.04007.x
r 2010 The American Ceramic Society
ournal
J
A Solvothermal Approach for the Preparation of Nanostructured
Carbide and Boride Ultra-High-Temperature Ceramics
James P. Kelly,* Raghunath Kanakala,* and Olivia A. Graeve*,w
Kazuo Inamori School of Engineering, Alfred University, Alfred, New York 14802
The use of a solvothermal process for the synthesis of tantalum
carbide (TaC) and lanthanum hexaboride (LaB6) powders in
fused-quartz test tubes is reported in order to demonstrate the
synthesis of these powders using thermal and chemical ignition
techniques and to prove that the process is of a self-propagating
high-temperature synthesis type, obviating the need for an
autoclave. X-ray powder diffraction showed phase pure powders
with crystallite sizes of 25 and B80 nm, while dynamic light
scattering showed average particle sizes of 97 and B130 nm, for
TaC and LaB6, respectively. The data demonstrates that the
powders have a very low level of agglomeration. Scanning elec-
tron microscopy shows that the TaC powders have a spherical
morphology, while the LaB6 powders have a mixture of cubic
and spherical morphologies.
understanding of this technique difficult. For purposes of this
study, solvothermal synthesis will be redefined to include the
synthesis in a heated solvent, but without pressure. Specifically,
the focus of this work is to demonstrate a unique solvothermal
synthesis approach where synthesis is performed in transparent
fused-quartz test tubes, rather than in an autoclave, and to
demonstrate the viability of using the technique to make carbide
and boride nanopowders such as tantalum carbide (TaC) and
lanthanum hexaboride (LaB6), while at the same time allowing
direct observation of the reactions taking place during the
process. This study confirms that the process is similar to a
self-propagating high-temperature synthesis (SHS) process,
obviating the need for an autoclave and long processing times,
and producing fine and unagglomerated powders.
II. Experimental Procedure
I. Introduction
Reactant mixing was performed in an argon-filled glove box.
Three types of samples were prepared: (1) thermally ignited
TaC, (2) thermally ignited LaB6, and chemically ignited LaB6.
For the case of the thermally ignited TaC synthesis, 5.57 g of
tantalum (V) chloride (99.8%, Alfa Aesar, Ward Hill, MA) and
0.56 g of carbon (Lampblack 101, Degussa, Parsippany, NJ)
were mixed using a mortar and pestle to achieve homogeneity.
The mixture was added to a fused-quartz test tube with 1.62 g of
lithium (granular, 99%, Sigma-Aldrich, St. Louis, MO) and
sealed with a rubber stopper. A 50-g batch, using 92.8 g of tan-
talum chloride, 9.3 g of carbon, and 27.0 g of lithium, was also
performed in a 600 mL stainless-steel beaker to demonstrate
scale-up. The stoichiometric equation for the process is
RANSITION metal carbides, such as tantalum carbide, are
some of the most refractory ceramics known (having melt-
T
ing temperatures up to B4000 K). Besides being refractory,
other beneficial properties include thermomechanical stability,
thermochemical stability, corrosion and wear resistance, good
creep and fatigue resistance, unique optical and electronic prop-
erties, high emissivity at elevated temperatures, and catalytic
characteristics.1–8 These materials are of particular interest in
the aerospace industry, where reentry conditions and internal
rocket conditions result in harsh environments with demanding
specifications. In addition to carbides, transition metal borides
are also relevant materials for aerospace applications. In the
case of zirconium diboride/silicon carbide composites, the
oxidation resistance can be significantly enhanced with addi-
tions of lanthanum hexaboride to provide outstanding behavior
for temperatures up to 24001C.9
Solvothermal synthesis for carbide materials has been inde-
pendently demonstrated by Hu,10 Gu,11 Ma,6 and their cowork-
ers. The technique is historically defined as synthesis in a
nonaqueous solvent by the application of temperature and pres-
sure and is done within the confines of an autoclave. This can
result in heavily agglomerated powders due to the long process-
ing times. Typical reactants include chloride compounds and
alkali/alkaline earth metals where the metals reduce the chloride
reactants and provide nascent seeds to form the desired mate-
rials, but other reactants, including elements, have been used to
replace chloride compounds in some instances. Due to the use of
an autoclave for the process, the reaction is not directly observ-
able. Lack of direct observation has made the fundamental
TaCl5 þ C þ 5Li ! TaC þ 5LiCl
The reactants were mixed to obtain the TaC product, but
with additional carbon and lithium in amounts of 3 ꢀ carbon
stoichiometry and 3ꢀ lithium stoichiometry to guarantee that
the process proceeded to the right.
A similar procedure was used for the synthesis of LaB6. Lan-
thanum (III) chloride hydrate (99.9%, Alfa Aesar), boron
(Sigma-Aldrich), and lithium granules were used. The amounts
of lanthanum (III) chloride hydrate, boron, and lithium for the
thermally ignited sample were 3.82, 2.02, and 0.97 g, respec-
tively, which correspond to 2 ꢀ boron stoichiometry and 3 ꢀ
lithium stoichiometry. For the chemically ignited sample, the
reactant amounts were the same except for the amount of boron,
which was reduced to 1.01 g, corresponding to 1 ꢀ boron
stoichiometry. The reaction stoichiometries were determined
according to the following equation:
R. Koc—contributing editor
LaCl3 þ 6B þ 3Li ! LaB6 þ 3LiCl
After mixing, the closed sample tubes were removed from the
glove box for ignition underneath a chemical fume hood. Before
ignition, the test tubes were hand rolled to achieve mixing of the
lithium granules with the rest of the reactants. The thermally
Manuscript No. 27863. Received April 15, 2010; approved June 25, 2010.
This work was financially supported by the grant from the National Science Foundation
under grant numbers CMMI 0645225 and CMMI 0913373.
*Member, The American Ceramic Society.
wAuthor to whom correspondence should be addressed. e-mail: graeve@alfred.edu
3035

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Vol. 93, No. 10
ignited samples were lowered into a tube furnace that was held
at a temperature of 548 K, just above the self-ignition temper-
ature of lithium, after loosening the rubber stoppers to allow for
outgassing. Once ignition occurred, the sample was immediately
removed from the furnace and allowed to air quench. For the
chemically ignited sample, the rubber stopper was removed from
the test tube. Water was added into the test tube until ignition
occurred and the sample was allowed to air quench.
After air quenching, the samples consisted of solid reaction
products surrounded by solidified lithium. The products were
removed from the test tubes by adding water and forming lith-
ium hydroxide, thus, freeing the reaction products. The products
were then poured into a 250 mL beaker. Enough water was
added to create a 100 mL suspension, which was magnetically
stirred for 15 min, followed by ultrasonication for 15 min, and
magnetic stirring for another 15 min. The suspension was cen-
trifuged at 11 000 rpm for 5 min, decanted, and allowed to dry.
The dried powders were lightly ground in a mortar and pestle to
prepare them for washing procedures.
For washing, the TaC samples were magnetically stirred with
100 mL of concentrated nitric acid (15.8N) for 30 min, followed
by ultrasonication for 30 min, magnetic stirring for another
15 min, and centrifuging at 11000 rpm for 5 min. Two water
rinses were performed by adding water to the centrifuge tubes
and ultrasonicating for 5 min before centrifuging again. A final
ethanol rinse was performed in a similar manner. After pour-
ing of the supernatant, the sample was allowed to dry in air for
24 h before mixing with a mortar and pestle and storing for
characterization. A modified washing procedure was used for
the 50-g batch of TaC. Three water washes, using a procedure
similar to the nitric acid wash, were performed before drying.
The washing procedure for the LaB6 samples is described
elsewhere.12
The samples were characterized by powder X-ray diffraction
on a Siemens D5000 instrument (Siemens, New York, NY) by
scanning from 151 to 851 2y using a step size of 0.041 2y and a
dwell time of 2 s after dispersing the powders onto a zero-back-
ground holder. Jade 8 software was used to estimate the average
crystallite sizes using the Williamson–Hall technique. For TaC,
the analysis software was also used to perform profile fitting and
unit-cell refinement to determine the lattice parameter. The
lattice parameter was used to determine the compound stoic-
hiometry according to the equation C/Ta 5 6.398aꢁ17.516.13
Dynamic light scattering (DLS) on a Nanotract ULTRA
instrument (Microtrac, Inc., Montgomeryville, PA) was used
to determine the particle size distribution of 0.01 g of powder
dispersed in 25 mL of deionized water. The DLS samples were
allowed to magnetically stir for 24 h and were ultrasonicated for
5 min before the measurements were taken. Each DLS mea-
surement consisted of an average of five 30 s runs as is recom-
mended by the instrument manufacturer and in conjunction
with ASTM standard E2490-09. Scanning electron microscopy
(SEM) was performed on a FEIt Quanta 200F instrument (FEI
Company, Hillsboro, OR) to observe particle morphology and
supplement particle and crystallite size analyses. SEM samples
were prepared by dispersing 0.01 g of powder into 25 mL of
acetone, magnetically stirring for 1 h, ultrasonicating for 10 min,
drop coating onto a silicon wafer, drying, and then carbon coat-
ing. Specific surface area (SSA) measurements were performed
using a Tristar 3000 BET surface area analyzer (Micromeritics,
Norcross, GA) after degassing in argon for 24 h at 423 K. Den-
sity measurements were performed by helium pycnometry using
an AccuPyc II 1340 (Micromeritics, Norcross, GA) and was
used with SSA measurements to calculate an average crystallite
size according to 6000/(SSA ꢀ r), where SSA is in m2/g and r is
the density in g/cm3.
tion can be achieved either thermally or chemically. To ther-
mally ignite a sample, the self-ignition temperature of the
lithium metal must be exceeded by the application of heat.
Chemical ignition can be achieved by reacting lithium with
water to produce lithium hydroxide and hydrogen. Either pro-
cess results in a strong exothermic reaction that propagates the
combustion wave. Once the system is ignited, molten lithium is
formed, which serves as the solvent for the rest of the reaction.
The temperatures achieved during ignition are well above the
decomposition temperatures of the chlorides (B513 K for TaCl5
and B1273 K for LaCl3 versus reaction temperatures as high as
B1653 K) and therefore the reaction to form TaC will proceed
from the elements. The reactions to form TaC and LaB6 from
their elements are exothermic, having adiabatic flame tempera-
tures of 2700 and 2800 K, respectively.14 The energy from the
reaction has the capability of driving the temperature of the
solvent up to, but not exceeding, the vaporization temperature
of the lithium until the reactants are depleted.
The higher the exothermicity of the reaction, the more likely
this process will work for a given system. Faster heating and
cooling rates can be achieved with this approach when com-
pared with synthesis in a thick-walled autoclave vessel. This can
inhibit grain growth during the process and prevent secondary
reaction/crystallization events from occurring during a dwell or
slow cool. Furthermore, the apparatus is simpler, costs much
less, and can accommodate larger batch sizes more readily than
an autoclave. To demonstrate the ease of scale-up a 50-g batch
was prepared in a larger container.
Figure 1 illustrates the X-ray diffraction patterns and crys-
tallite sizes for the three small-scale samples. Results show that
single-phase TaC and LaB6 samples were obtained and the
average crystallite sizes were 25, 84, and 81 nm for the TaC,
thermally ignited LaB6, and chemically ignited LaB6, respec-
tively. Phase purity is obtained even in the case of the chemically
ignited LaB6. This is significant because this sample was exposed
to oxygen from dissociation of water molecules as well as from
the fact that the sample stopper was removed before the reac-
tion, which means that synthesis is essentially performed in air.
A minor amount of lanthanum borate was formed as a conse-
quence of this, but was removed during the washing procedure.
The source of ignition did not result in a difference in the
average crystallite size of the LaB6 powders, suggesting that
the mechanism for the synthesis is the same regardless of how
the reaction is started and further supports the idea that the
reaction proceeds by self-propagating synthesis from the ele-
ments. The TaC compound stoichiometry, determined by lattice
refinement, was 0.94. The 50-g batch was nearly phase-pure,
III. Results and Discussion
Direct observation of the solvothermal reaction suggests that it
is a fast, self-sustaining reaction, requiring only ignition. Igni-
Fig. 1. X-ray diffraction patterns for the TaC and LaB6 powders (d is
the crystallite size).

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Rapid Communications of the American Ceramic Society
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containing a minor amount of tantalum metal. The decrease in
phase purity is believed to be a result of mixing, where localized
regions were carbon deficient, and indicates that process opti-
mization is required. The crystallite size for the large batch-size
was 25 nm and the compound stoichiometry was 0.94 indicating
no change from the smaller batch size.
Figure 2 illustrates the DLS results for the three small-scale
samples. TaC has an average particle size of 97 nm with an
upper-tail distribution that falls off rapidly (the majority of par-
ticles are below 200 nm). Similarly, the majority of particles are
below 200 nm for the large-scale batch, but the average particle
size was 73 nm, indicating a small improvement in the particle
size distribution. The average particle sizes of the two LaB6
samples have a small bimodal peak in the upper-tail distribu-
tion. The second mode occurs around 500–600 nm and is more
prevalent in the chemically ignited sample. A useful figure of
merit to characterize the amount of hard agglomeration is a
ratio between the average particle size and the average crystallite
size. These figures of merit are 3.9, 1.6, and 2.0, for the TaC,
thermally ignited LaB6, and chemically ignited LaB6, respec-
tively, which suggests a very low state of agglomeration for the
powders. The figure of merit for the 50-g TaC batch is 3.0,
indicating that scaling of the process actually assists in achieving
a lower level of agglomeration. Ma et al.6 reported a crystallite
size of 40 nm for their TaC powders from SEM imaging, noting
that the powders are agglomerated due to their fine size (particle
necking between particles is clearly evident from the images, in-
dicating significant hard agglomeration. Gu et al.11 report a
crystallite size of 15–40 nm and slight agglomeration from TEM
analysis. Neither analysis presents the true particle size to obtain
a figure of merit. Thus, we propose from the visual data that
both sets of powders are heavily agglomerated due to the long
processing times and slow cooling from the use of an autoclave.
SEM for the three samples are shown in Fig. 3. TaC has a
near-spherical morphology and it is clear from the images that
the individual grains are very small, in agreement with the crys-
tallite and particles size results. The LaB6 samples appear to
have a mixture of cubic and near-spherical morphologies. The
cubic faces in LaB6 correspond to the lower energy {100} planes
of the cubic crystal structure in this material.15 The cubic mor-
phology arises during washing, but the presence of near-spher-
ical morphologies indicates the mechanism is not complete
under the current washing procedure.12 Larger grains up to
1 mm are evident for the LaB6 samples, but the majority of
grains appear to be in submicrometers and below 300–400 nm,
which agrees well with the upper limit of the left-hand side of the
Fig. 3. Backscattered scanning electron micrographs of (a) TaC,
(b) thermally ignited LaB6, and (c) chemically ignited LaB6.
curve in the bimodal distribution of the DLS results. The larger
grains are associated with the upper-tail peak of the right-hand
side of the curve in the bimodal distribution, which is centered
around 500–600 nm with the upper limit approaching 1 mm.
Darker patches that blur portions of the LaB6 images in certain
locations are most likely the result of either residual boron or
lithium that was not fully removed during the washing proce-
dure and would require further optimization to eliminate. Dis-
tinct particle necking is not clearly observed and the images
complement the crystallite size and particle size distributions
obtained from XRD and DLS.
The SSA for the small-scale and large-scale synthesis of the
TaC samples were 39.9 and 19.5 m2/g, respectively, while the
measured densities were 7.6 and 9.7 g/cm3, respectively, much
lower than the theoretical density of 13.9 g/cm3. The estimated
average crystallite sizes using the surface area and density data
Fig. 2. Particle size distributions for the TaC and LaB6 powders (D is
the number-average particle size).

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Acknowledgments
Vol. 93, No. 10
were 20 and 32 nm, respectively, in good agreement with the
value of 24 nm determined by X-ray diffraction. The SSA of the
thermally ignited and chemically ignited LaB6 samples were 6.6
and 7.0 m2/g, respectively, while the density measurements were
2.9 and 3.0 g/cm3, and also much lower than the theoretical
density of 4.7 g/cm3. The estimated average crystallite sizes from
this data are 313 and 285 nm and are larger than the average
crystallite sizes determined by X-ray diffraction (84 and 81 nm,
respectively) as well as larger than the average particle sizes
determined by DLS (131 and 162 nm, respectively), elucidating
the difficulty of extracting particle or crystallite size information
from SSA and density measurements. In the case of the LaB6
density values, the large discrepancy may be a result of the low-
density phase that was observed in Fig. 3. The discrepancies in
the SSA, density, and average crystallite size data require further
evaluation and will be presented in a future communication.
Likewise, carbon and oxygen analysis cannot be performed until
the state of the surfaces after the washing procedure is better
understood.
This project was funded by a grant from the National Science Foundation
under grant numbers CMMI 0645225 and CMMI 0913373.The authors gratefully
acknowledge the assistance of Mr. Braeden Clark in completing the large-scale
TaC synthesis experiments.
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